Halide Perovskite: Observing Phase Transitions in a Halide Perovskite
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Halide Perovskite Phase Transitions Observations Using Temperature Dependent Photoluminescence Spectroscopy

Hybrid halide perovskite has recently emerged as a new class of low cost semiconductor for optoelectronic applications. They have received widespread attention as an absorber in photovoltaic cells, where they have achieved a remarkable increase in solar cell efficiency; from 9% in 2012 to 21% in 2017.

Despite this high efficiency, further research into the fundamental physics and stability of hybrid halide perovskite is required. One important area of research is understanding how temperature affects the properties of the halide perovskite. Photovoltaic cells are exposed to a wide range of climates, with ambient temperatures ranging from -20°C to 40°C. Furthermore, the actual working temperature inside the cell could greatly exceed the ambient temperature due to heating from solar irradiation, and has been shown to be as high as 70°C in some cases. It is therefore crucial to understand the influence that temperature has on the photophysics and performance of perovskite photovoltaic cells.

The FLS1000 Photoluminescence Spectrometer can be equipped with a range of cryostat accessories that enable the photoluminescence properties of a material to be studied over a temperature range of 2.3 K to 800 K. In this application note the temperature dependence of the photoluminescence of methyl ammonium lead iodide perovskite is investigated using the FLS1000 with the liquid nitrogen cryostat accessory.

MAPI Halide Perovskite Crystal Structure Cubic Tetragonal Orthorhombic

The cubic, tetragonal and orthorhombic crystal structures of MAPI halide perovskite. The phase that the perovskite adopts depends on the temperature of the lattice. Transitions between these three phases can be measured using photoluminescence spectroscopy.

The term perovskite refers to any compound which has an ABX3 crystal structure, where A and B are two different cations and X is the anion. The highest performing and most widely studied perovskite for photovoltaic applications is methylammonium lead iodide (CH3 NH3 PbI3), also known as MAPI, the structure of which is shown in Figure 2.

The lead cation (blue) is located in the centre of a cube of methylammonium cations (green) and is surrounded by an octahedron of iodide anions (red). There are three different phases of the crystal structure that MAPI can adopt, with the temperature controlling which phase is thermodynamically stable. At room temperature MAPI is known to be in the tetragonal phase where two of the unit cell lengths are equivalent (a = b ≠ c).At lower temperatures the tetragonal symmetry is broken and MAPI adopts the orthorhombic structure where all three unit cell lengths are different (a ≠ b ≠ c).5  At elevated temperatures the symmetry of the structure increases, with MAPI adopting the cubic crystal structure (a = b = c).5  The temperature of the transitions between these three structures can be determined using photoluminescence spectroscopy.

Methods and Materials

MAPI perovskite films were deposited on quartz discs, using the methylamine / acetonitrile route developed by Noel et al. 6  After deposition of the perovskite, the films were annealed on a hotplate at 100°C for 10 minutes. Photoluminescence emission spectra were measured on an FLS1000 Photoluminescence Spectrometer equipped with double monochromators, a 450 W Xenon lamp, a PMT-980 detector and an extended temperature range (77 K to 500 K) liquid nitrogen cryostat (N-K06exd). The perovskite coated quartz discs were secured inside the cryostat and photoluminescence measurements performed under a nitrogen atmosphere. The sample was allowed to equilibrate for 10 minutes between each temperature change. The cryostat temperature was fully controlled using the Fluoracle software of the FLS1000. This allowed the photoluminescence temperature maps to be acquired automatically by Fluoracle, without requiring oversight by the user.

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